The invention relates to the general field of microfluidics with particular reference to electrically isolatable micro-channels.
Micro-machined capillaries have found wide application in biological, pharmaceutical, and chemical applications. They also play an important role in micro-fluidic devices. They can be formed in various materials but, because of bio-compatibility considerations, surface hydrophilic properties, good electrical isolation, general chemical inertness, and good optical transparency, glass materials are often preferred, increasing the importance of being able to fabricate glass capillaries or capillaries having a glass internal surface.
The conventional approach to fabricating glass capillaries has been glass-to-glass fusion bonding. This requires special surface cleaning and treatment temperatures in the 500-700xc2x0 C. range, which has a negative impact on process yield. To avoid the fusion bonding process, the anodic bonding process has been developed for glass to silicon dioxide bonding (typically on top of a silicon substrate). However, because silicon is opaque and the thickness of the oxide layer used for the bonding is low, straightforward application of anodic bonding limits the range of applications for its use. Another important limitation is that the thin oxide provides limited isolation from the electrically conductive silicon substrate so that only relatively low voltage differences between adjoining capillaries can be sustained.
Several methods to achieve a capillary with a glass internal surface have been described in the prior art. Fusion bonding of two glass plates, one of the plates having prefabricated channels on the internal surface, is one example of the prior art (D. J. Harrison, A. Manz, Z. Fan, H. Ludi, H. M. Widmer, Anal. Chem. 64(1992)1926-1932). A direct bonding technique was also developed in which the glass substrate and the cover plate were first surface hydrolyzed and then joined by hydrogen bonding, followed by annealing at 500xc2x0 C. (S. C. Jacobson, R. Hergenroder, L. B. Koutny, R. J. Warmack, J. M. Ramsey, Anal. Chem. 66 (1994) 1107-1113). Anodic bonding of an oxidized silicon channel wafer to a glass plate has already been mentioned. Sodium silicate or potassium silicate bonding of glasses is a technique for forming a capillary with a glass internal surface using a low temperature process. However, as was the case for glass fusion bonding techniques, surface pre-cleaning and air bubble removal are very critical for achieving good bonding quality.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 4,908,112, Pace shows a wafer for analyzing biological samples using a capillary conduit. Glass to glass bonding by a polyamide gasket method has also developed by Pace and others. However, the internal surfaces for such types of capillary are of different materials, which may cause micro fluidic problems.
In U.S. Pat. No. 5,824,204, Jerman shows a micro-machined capillary device. Anodic bonding is used for bonding planar glass to a micro-fabricated silicon channel wafer with a partially oxidized, low stress nitride channel surface. After anodic bonding, the silicon is etched away. leaving glass and a partially oxidized nitride structure. This method has the advantages of good optical and electrical isolation properties because the silicon has been etched away. However, after the silicon has been removed, all that remains is a very thin partially oxidized low stress nitride film structure having three walls that form the channel. This makes for a fragile device structure, even if additional protection by polyamide is included.
Mathies et al. in U.S. Pat. No. 6,261,431 b1 show a process for a PCR-CE device. U.S. Pat. No. 6,224,830 b1 (Harrison et al.) shows an absorbance cell for micro-fluidic devices. U.S. Pat. No. 6,176,962 b1 (Soane et al.) shows an enclosed micro-channel structure method U.S. Pat. No. 5,376,252 (Ekstrom et al.) also shows a micro-fluidic structure.
The method to fabricate capillaries with glass internal surfaces that is disclosed in the present invention offers advantages such as good electrical isolation and optical properties as well as a lower process temperature relative to prior art processes.
It has been an object of at least one embodiment of the present invention to provide a process for manufacturing a capillary that is electrically insulated from its environment.
Another object of at least one embodiment of the present invention has been that said capillary be visible for optical inspection.
Still another object of at least one embodiment of the present invention has been that said capillary be mechanically robust.
A further object of at least one embodiment of the present invention has been to provide a process for manufacturing a capillary that possesses these qualities.
These objects have been achieved by means of a process having two stages. The first stage, which is the same for both of the invention""s two embodiments, comprises forming a micro-channel in the surface of a sheet of glassy material. For the first embodiment, this sheet is bonded to a layer of oxide, that lies on the surface of a sheet of silicon, thereby sealing in the capillary. After all silicon has been selectively removed, a thin membrane of oxide remains. Using a low temperature bonding process, a second sheet of glassy material is then bonded to this membrane. In the second embodiment, the silicon is not fully removed. Instead, the oxide layer of the first embodiment is replaced by an oxide/nitride/oxide trilayer which provides good electrical insulation between the capillary and the remaining silicon.